We have produced a unique reagent which we feel has great promise as a tool for the study of the mechanism of platelet aggregation. The mouse monoclonal antibody UMR-304 inhibits the ADP, epinephrine, and thrombin-induced second wave of aggregation of human platelets. UMR-304 also blocks aggregation induced by collagen and thromboxane A2 in a dose dependent fashion. It does not block the formation of thromboxane A2 by dog platelets stirred with arachidonic acid. Serologic and immunochemical data indicate that this antibody reacts with Beta-microglobulin (Beta2m), a highly conserved surface molecule that is found on all nculeated cells in association with the allotypic chains of the HLA antigen system. Beta2m also associates with other non-HLA structures which are thought to represent differentiation antigens coded by the major histocompatibility locus. A second monoclonal antibody with simolar anti-aggregating activity has been identified from a panel of clones derived from mice immunized with purified Beta2m. These data suggest several possibilities: 1) Beta2m containing complexes such as HLA are directly involved in platelet aggregation, 2) binding of antibody to Beta2m complexes sterically hinders binding of other mediators to adjacent receptors or 4) UMR-304 corss reacts with another as yet undetermined surface protein critical to aggregation that shares antigenic sites with Beta2m. Using UMR-304 as a tool in immunochemical techniques, we plan to isolate and characterize the surface structure responsible for its effect on platelet function. In parallel experiments, we will examine directly the effect of UMR-304 on the binding of platelet activators, the release of platelet factors in the second wave, and the production of products of the cyclooxygenase pathway. When isolated, the activity of the UMR-304 reactive structure will be confirmed by its use as a competitive inhibitor, and through the action of new antibodies produced against the isolated antigen. This work has the potential to uncover a relationship between the major histocompatibility locus and platelet function and should shed light on the precise mechanism of the second wave of platelet aggregation.